Cytotoxicity of cancer cells grown under methionine deprivation has been well established. Depletion of methionine induces death of cancer cells, whereas normal cells are much more resistant. The reasons for this relative selectivity are unknown. Normal and tumor cells can synthesize methionine provided sufficient folate, cobalamin, and homocysteine are supplied. However, most tumor cells require larger amounts of methionine than what they can synthesize, and, in the absence of an exogenous supply, they experience growth inhibition or die.
Depletion of methionine in vitro and in vivo can be achieved through the action of methionine gamma-lyase (L-methionine-α-deamino-γ-mercaptoethane lyase; MGL) that catalyses irreversibly the α,γ-elimination of L-methionine resulting in production of methanethiol, α-ketobutyrate, and ammonia. Various MGLs have been produced by purification from several microorganisms or by recombination of genes encoding for the enzyme originating from various bacterial species and protozoa. Most studies on the antitumor action of methionine depletion have used the MGL from the potentially pathogen pseudomonade, Pseudomonas putida (Pp-MGL).
Methionine depletion obtained with Pp-MGL enhances the cytotoxic action of the fluoropyrimidine 5-fluorouracil (FUra), an agent currently used for the treatment of various types of human cancer (Machover et al., 2001; Machover et al., 2002). Potentiation is due to modifications of the intracellular pools of reduced folates induced by methionine depletion and, possibly, through changes in the expression of cellular mechanisms favouring cell death, which may be related to DNA demethylation (Machover et al., 2001; Machover et al., 2002). Other investigators have demonstrated potentiation of FUra and cisplatin in tumor-bearing mice by simultaneous administration of Pp-MGL.
Animal experiments have been performed with the aim to introduce recombinant Pp-MGL in the clinics. However, lethal anaphylactic shock syndromes have been observed when monkeys were re challenged with the protein, which has prevented further development of the native recombinant Pp-MGL. Attempts at reduction of the immunogenicity of the protein through pegylation have not succeeded at the present time.
This pre clinical data believed that the prohibitive immunogenicity of the P.p.MGL, and possibly that of other putative and well characterized MGLs described thus far, which derive all from various micro organisms that are potential pathogens for humans (i.e., originating from the bacteria Aeromonas sp., Citrobacter freundii, Porphyromonas gingivalis, and Treponema denticola, and the protozoa Trichomonas vaginalis, and Entamoeba histolytica), could not allow intra venous administration of the enzyme required for sustained methionine depletion in plasma under safe conditions.
However, there is a need to develop new drugs against cancer. In this way, it has been suggested that characterisation of new therapeutic targets inhibiting tumor cell growth may be highly desirable. There is thus a need in the art for methionine gamma-lyase derived from a non pathogenic micro organism abundantly present in food, this MGL may benefit from oral immune tolerance allowing its administration into the blood stream in a subject in need thereof for treatment of cancer.